Method and lens system for modifying the modulation transfer function of light for a camera

ABSTRACT

A method and lens system for modifying the modulation transfer function of the light passing through the lens system to a camera or other imaging system that includes providing a spatial frequency response modifying (SFRM) filter within the lens system. The SFRM filter may be a thin film(s) or other type of modifying filter, including any conventional diffusion filters. The SFRM filter is located along the optical axis where the on-axis, zero field angle light rays are collimated in any lens system and form light beam(s) of a substantially constant cross sectional area throughout the range of focusing of a prime objective lens or throughout the ranges of zooming and focusing of an objective zoom lens for providing a consistent modulation transfer function of the light reaching the camera or other imaging system.

CROSS-REFERENCE TO RELATED APPLICATION

This application is related to application Ser. No. 09/447,837, filed onNov. 23, 1999, which application is specifically incorporated herein, inits entirety, by reference.

The present invention relates to an optical lens system and method for acamera or other imaging system and, in particular, to such a lens systemand method for modifying the modulation transfer function of the lightsupplied to the camera or imaging system in a predetermined manner forany desired purpose. The present invention is particularly advantageouswhen used with electronic cameras, such as modern high-definitiontelevision (“HDTV”) cameras, but may also be used with other videocameras, film cameras and imaging systems.

Recent advances in Charge Coupled Device (CCD) technology have enabledelectronic cameras to be built having resolution capabilities whichrival conventional 35 mm motion picture film and it is likely thatfurther developments will improve the resolution capabilities evenfurther. Currently, HDTV video cameras are being developed using highresolution CDDs (approximately 2 million pixels per color) which willoperate at the nominal frame speed of a motion picture camera (24 framesper second). These cameras are being developed, in part, as replacementsfor film cameras, at least for some applications. However, for theforeseeable future, film and digital cameras will need to coexist in ahybrid production environment, which will exploit the uniquecapabilities of both imaging mediums.

Modern motion picture film imaging utilizing an image size ofapproximately 18 mm by 24 mm on so-called 35 mm film is the result of anevolution of science, art and craft which has taken place over a periodof 100 years. Any new imaging technology, such as HDTV, must be able tointegrate within this established aesthetic paradigm. However, modernHDTV utilizes a so-called ⅔″ CCD which is only 5.37 mm by 9.60 mm with adiagonal of only about 11 mm, compared to a 35 mm motion picture filmframe diagonal of about 27.5 mm, and yet for cine movie theaters the twoformats are projected onto the same sized large screen, whereby the HDTVimage is magnified about 2.5 times more than 35 mm film. This can createmore visible changes or flaws in the projected picture from an HDTVcamera than a film camera using conventional filming techniques anddevices.

For high quality motion picture productions, such as theater movies, itis a common practice to use so-called diffusion filters for achievingcertain desired effects by modifying the spatial frequency response,i.e., the resolution of the light. A typical diffusion filter scattersthe light, and the magnitude of scattering can be established byselecting a particular filter for producing the desired effect. Forexample, one diffusion filter may be used to create a blurred appearancefor a scene and another diffusion filter may be used to eliminate thefine lines on the face of a mature actor or actress in a close-up. Bothsuch filters serve to modify the so-called resolution, which is thespatial frequency response (commonly measured as a percent of spatialfrequencies in cycles per mm, of the light supplied to the camera and agraph of such values represents the modulation transfer function).Typically, such conventional diffusion filters have been used to reducethe response in the higher cycles/mm.

Diffusion filters are commonly mounted on the front of the objectivelens, although some can be mounted on the back depending on the spacelimitations that may exist. Conventional diffusion filters are assophisticated as special, multi-layer thin film coatings on opticalglass or as basic as a silk stocking stretched over the lens, whichstocking is particularly suitable for the back of the lens because thestocking doesn't require any additional space and can be secured merelyby the lens mount fitting between the lens and camera. Any diffusionfilter mounted on the back of the lens requires removal of the lens forremoving or changing the filter, which may be an objectionable extrastep, particularly for currently popular zoom lenses that may be large,heavy and normally do not require changing for the entire shooting of ascene or scenes. On the other hand, a diffusion filter for mounting onthe front of a wide-angle lens or a zoom lens may be very large andtherefore may be very expensive.

Another problem with conventional diffusion filters is that the visualeffect changes with changes in focus and, in the case of a zoom lens,also with changes in the focal length, i.e., zooming. Thus, for example,while a particular diffusion filter mounted on the front of an objectivelens may effectively cause wrinkles on the face of a mature actor oractress to be invisible on the film or CCD at one focus distance or onezoom focal length or both, a change in the focus distance or focallength or both may objectionably cause those wrinkles to reappear oreven make other desirable lines or features become invisible or lessvisible. Such changes are visually unacceptable in a high quality moviefor projection on a screen in theaters.

These and other problems with the use of diffusion filters areaccentuated with an HDTV camera because of the large magnificationrequired from the ⅔″ CCD to a theater screen, i.e., 2.5 times that of 35mm film, as described above. In other words, the film is operating at aspatial frequency of 20 cycles/mm while the CCD is operating at 50cycles/mm whereby the problem is amplified by 250%. Thus, for example,even the variations or imperfections in the diffusion filters that arecommercially available at present may affect the image captured by theCCD. Further it is very difficult, if not impossible, to obtain the samespatial frequency response, i.e., resolution, in both a film camera andan HDTV camera for movie productions that use film and videointerchangeable, which is becoming popular. For example, the 2.5 factorbetween 35 mm film and a ⅔″ CCD does not directly translate into theunits of strength of diffusion filters, i.e., a “2.5” unit diffusionfilter on the film camera doesn't produce the same effect as a “1.0”unit diffusion filter on the CCD. In fact, even the variations betweendiffusion filters of the same type and units from the same manufacturercan produce substantially different results, particularly in the HDTVcamera because of the increased sensitivity due to the increasedmagnification versus film.

Therefore, it is an object of the present invention to provide a methodand a lens for use with a camera or other imaging system for modifyingthe modulation transfer function of the light passing through the lensto a predetermined relationship between the spatial frequency responseand modulation of the light being supplied to the camera for causing thecamera or other imaging system to record a desired and consistentspatial frequency response over the entire range of focus or focallength (zoom) adjustments.

In the preferred embodiment of the present invention, the objective lensis provided with an optical filter means on the optical axis at alocation where the on-axis light rays form a light beam of asubstantially constant cross-sectional area throughout focusing orzooming changes or both, which location preferably will be where thelight rays are substantially collimated, for producing a predeterminedmodulation transfer function for the camera. Preferably, that opticalfilter means is of zero optical power for practical reasons, such asinterchangeability, lower cost and minimizing the optical effectthereof. When the optical filter means is made of an opticallytransmissive material, such as glass, plastic, liquid or the like,preferably it is replaceable with a comparable optical element havingsubstantially no spatial frequency response modifying effect or, whenmade of a non-optical material which passes some light, it can beremoved so that there is substantially no spatial frequency responsemodification of the light to be supplied to the camera. A further objectof this invention is to provide an objective lens with such an opticalfilter means that produces a predetermined modulation transfer functionof light supplied to a camera that can match a specific modulationtransfer function of light recorded on film, or, in other words, matchthe modulation versus spatial frequency of the video and film, asobserved on a critical presentation medium, such as a large cinemasystem.

Other objects, advantages and features of the present invention willappear from the detailed description hereinafter of preferredembodiments in connection with the accompanying drawings, wherein:

FIGS. 1A, 1B and 1C are optical diagrams of an objective lens of thezoom type having a variable focal length with the present inventionincorporated in the lens, and illustrating the lens elements in threedifferent zoom positions;

FIG. 2 is an enlarged view of a portion of the optical diagram of FIG.1C;

FIGS. 3A, 3B and 3C are optical diagrams of the zoom lens of FIGS. 1A,1B and 1C in the same three zoom positions but with the spatialfrequency filter element for the present invention positioned at adifferent location along the optical axis;

FIG. 4 is an enlarged view of a portion of the optical diagram of FIG.3C;

FIGS. 5A and 5B are optical diagrams of the same zoom lens in the samezoom position as FIG. 3C and with the diffusion filter in the samelocation as FIG. 3C, but with the focus grouping of lens elementspositioned at the two extremes, i.e., at infinity focus and at closefocus, respectively;

FIGS. 6A and 6B are optical diagrams of a prime (fixed focal length)lens with the focus group of lens elements positioned at the twoextremes, i.e., at infinity focus and at close focus, respectively;

FIG. 7 is a diagrammatic elevation view of a video camera having a relaylens system and incorporating the present invention;

FIG. 8A is an optical diagram of the relay lens system of FIG. 7 in thephysically folded arrangement of FIG. 7;

FIG. 8B is an optical diagram of the relay lens system of FIG. 7 butwith the optical axis unfold into a straight line for illustrationpurposes; and

FIG. 9 is a graph of the Modulation Transfer Function (MTF), which isthe modulation (so-called “contrast”) in percent versus the spatialfrequency (so-called “resolution”) in cycles/mm for various arrangementsand filters for illustrating a variety of spatial frequency responsesthereof.

The present invention will be described in connection with two differenttypes of high performance lenses, namely, a high performance zoom lensshown in FIGS. 1A through 5B, and a high performance prime (fixed focallength) lens shown in FIGS. 6A and 6B, which lenses are of a type andquality for use on cameras in cinematography, high definitiontelevision, advanced television and the like. The zoom lens of FIGS.1A–5B and the prime lens of FIGS. 6A, 6B are objective lenses thatinclude the filter element and method of the present invention in anappropriate manner, but otherwise are conventional objective lenses,which demonstrates that the invention is applicable to any type ofconventional objective lens. Also, the present invention will bedescribed in connection with another type of imaging system, namely, arelay lens shown in FIGS. 7, 8A and 8B for relaying an image from anobjective lens to an image detector. These embodiments also illustratethat the invention is applicable to any optical imaging system, not onlyobjective lenses, such as a relay lens and others. Moreover, theinvention may be beneficial in some unusual instances by specificallydesigning an objective lens or other lens system with appropriatecharacteristics for using the present invention. While the invention hasand will be described herein with respect to lens systems for cameras,either film or electronic, it will be understood that the invention isapplicable to any imaging system, all of which will be referred to ascameras.

Referring now to FIGS. 1A–1C, the zoom lens 10 has the requisite groupsof lens elements including a stationary objective lens group S, amoveable focus lens grouping F, a moveable zoom lens grouping Z, and astationary collecting lens group C (sometimes called an “auxiliary” or“relay” lens), aligned on an optical axis O in that order from the frontof the lens near object space to the rear of the lens at the image plane12 (i.e., from left to right as viewed in FIGS. 1A–1C, as well as theother Figs.). The illustrated zoom lens 10 has a focal length range fromabout 6 mm to 27 mm, but the present invention is applicable to a zoomlens of any range of focal lengths, as well as any fixed focal length(prime) lens and other lens systems. Further, the illustrated zoom lens10 has two lens groups forming the focus grouping F and two other lensgroups forming the zoom grouping Z, each of which lens groups aremovable relative to each other for accomplishing focusing at allappropriate distances and zooming for the full range of focal lengths,but more or fewer lens groups could be used to perform those functions.An optical stop or iris 14 is located immediately in front of thecollecting lens group C. FIGS. 1A–1C and 2 illustrate a zoom lensoptimized for an HDTV camera and therefore also illustrate prisms P1 andP2 on optical axis O, which prisms are included in the HDTV camera, forcompleting the optical diagram to the image plane 12, but zoom lens 10could be modified to be optimized for a film camera.

A representative number of light ray tracings T1–T5 are shown in theoptical diagrams of FIGS. 1A through 6B for illustrating the light raypaths and the angles of incident of such light ray paths at the variousoptical elements, the importance of which will be discussed below. Lightray tracings T1 and T2 are “on-axis” (zero field angle) tracings that,at infinity focus, are parallel to and symmetrical about optical axis Oas they enter the lens 10 on the left side, and T3–T5 are typical“off-axis”(non-zero field angle) light ray tracings that are at an angleto the optical axis O as they enter the lens 10. As thus far describedwith respect to FIGS. 1A–1C, the zoom lens is relatively conventionaland these conventional portions are the same for the zoom lens of theother FIGS. 2 through 5B.

Referring also to FIG. 2, which is an enlargement of the portion of FIG.1C illustrating the elements between the iris 14 and the image plane 12,an optical element comprising the spatial frequency response modifyingfilter 16 a (hereinafter referred to as the “SFRM filter”) portion ofthe present invention is positioned on the optical axis O among the lenselements forming the collecting lens group C. Preferably, the SFRMfilter 16 a is removable and replaceable from externally of the lenshousing (not shown) that supports the lens elements illustrated in theoptical diagrams of FIGS. 1 and 2, whereby the filtering characteristicsof the SFRM filter 16 a may be varied or eliminated to meet the desiredresults. If the SFRM filter 16 a is removable and replaceable, then aclear substrate of the same material and thickness may be substitutedwhen no modification of the light rays is desired, such as when thecamera is being used for conventional purposes. The SFRM filter 16 a maybe comprised of any substantially transparent material and be providedwith any means that serves or may serve to modify the ModulationTransfer Function (MTF) of the light passing therethrough, such as, butnot limited to, a conventional glass diffusion filter, and various otherspecific examples of suitable SFRM filters that will be described below.

Referring again to FIGS. 1A–1C and 2, the location of the SFRM filter 16a in a lens is specifically selected or designed into the lens to bepositioned along the optical axis O at a point where the on-axis lightrays that will form the image on image plane 12 comprise a light beam ofa cross sectional area that remains substantially constant during all ofthe operating adjustments of the lens, including zooming and focusingover the full range of those adjustments for that lens or opticalsystem. By selecting a location along the optical axis of the lens wherethe on-axis light rays form a light beam cross sectional area or sizethat remains substantially constant, the modulation transfer function bythe SFRM filter 16 a will remain substantially constant throughout thezooming and focusing adjustments of the lens because the density,concentration, effectiveness and the like of the means, elements,coatings, thin films or the like that cause the spatial frequencyresponse modification that are located in the usable beam of lightforming the final image remain substantially identical and thereforeproduce a substantially constant modulation transfer function. Suchlocation normally is also where the light rays are substantiallycollimated, i.e., substantially parallel, which is preferred but notabsolutely required. Also, the location must not interfere with theother optical elements or functions of the lens, such as focus and zoomadjustment movements. It should be noted that the off-axis light raysare subject to vignetting, as is well known to those skilled in the art,and therefore, and while the cross sectional area of the light beamsformed by the unvignetted off-axis light rays will remain substantiallyconstant, the cross sectional area of the light beams formed by thevignetted light rays will change, but this change does not significantlyaffect the desirable results of the present invention.

As illustrated in FIGS. 1A–1C and 2, the SFRM filter 16 a is locatedwithin a space among the optical elements of the collecting lens group Cwhere the light ray tracings T1–T2 are symmetrical about and nearlyparallel to the optical axis O as those tracings pass through the SFRMfilter 16 a. Although the substantially constant cross sectional area atSFRM filter 16 a of the light beams forming the image at image plane 12is more important to the present invention for maintaining asubstantially constant modulation transfer function, some types of meansfor creating spatial frequency response modification may perform moreeffectively when the angle of incident of light on the SFRM filter 16 ais as close to perpendicular as possible, such as at this location.Thus, since SFRM filter 16 a preferably is flat and perpendicular to theoptical axis O, preferably the light ray tracings across the entireeffective surface of the optical element 16 a are nearly parallel to theoptical axis O, as shown by tracings T1 and T2. In the zoom lens 10 ofFIGS. 1A–5B, the angle between the flat surface(s) of optical element 16a and the on-axis light rays is not exactly 90°, except on the opticalaxis O, and is not the same across the entire surface of optical element16 a, but rather the angle varies slightly. Further, the location ofSFRM filter 16 a is such that the focusing adjustments and zoomingmovements of the lens 10 do not significantly change the ray incidenceangles or the cross sectional area of the on-axis light beams at element16 a.

By comparing FIGS. 1A, 1B and 1C, which illustrate the minimum, mediumand maximum focal length positions, respectively, of the zoom lens 10,it may be observed that the two light beams represented by the raytracings T1–T5 remain substantially the same size, with the on-axistracings T1, T2 virtually the full size, at the SFRM filter 16 a,whereas the light beam size (cross sectional area) changes substantiallyat other forward (left of iris 14) locations within the zoom lens 10,among the three positions, which changes in area would change thespatial frequency response if the SFRM filter 16 a was located at suchother locations.

As an alternative to providing a separate SFRM filter 16 a with meansfor creating spatial frequency response modification, if the lens 10 isdesigned for a single purpose, such as always simulating a specificfilming effect, a spatial frequency response modifying thin film(s)coating or the like may be provided on the surface(s) of one or more ofthe other optical elements in the lens 10. Preferably, such surface(s)is substantially perpendicular to the on-axis light ray tracings, suchas surface 17 in FIG. 2, and even though such a surface is not opticallyflat, it is acceptable if such surface is located where the crosssectional area of the on-axis light beam(s) remain substantiallyconstant and the element preferably is removable and replaceable.

Referring now to FIGS. 3A–3C and 4, the high performance zoom lens 10illustrated in these figures is the same as the zoom lens 10 illustratedin FIGS. 1A–1C and 2 with the only difference being in the location ofthe SFRM filter optical element. Thus, all of the lens groups and otherelements are the same in FIGS. 1A–4 (as well as in FIGS. 5A and 5Bdescribed below) and will not be specifically described with respect toeach illustration of the embodiments. In this embodiment of FIGS. 3A–4,the SFRM filter optical element 16 b, comparable to optical element 16 ain the first embodiment, is located at the front of the collecting lensgroup C and immediately behind the iris 14 as another example of adesirable location. Again, optical element 16 b is preferably anoptically flat element provided with any means for modifying the lightfor producing the desired predetermined spatial frequency response ofthe light that reaches the camera at the image plane 12. Preferably, theoptical element 16 b is removable and replaceable with other filterelements for producing other predetermined modifications of the spatialfrequency response or other effect on the light, including clear glassor other substrate that will not modify the light reaching the camera orchange the optical characteristics of the lens. The location of opticalelement 16 b in this second embodiment is particularly advantageousbecause the on-axis light rays (tracings T1 and T2) are substantiallyparallel to the optical axis and the off-axis light rays (tracingsT3–T5) are parallel to each other. Another advantage may be that theelement is smaller in diameter and therefore less costly. FIGS. 3A, 3Band 3C illustrate short, medium and long focal lengths of the zoom lens10, as in FIGS. 1A, 1B and 1C, respectively, and a comparison of FIGS.3A, 3B and 3C again demonstrates that the on-axis beams of light at SFRMfilter 16 b maintain a substantially constant cross sectional area,whereby the spatial frequency response remains substantially constant.It should be noted that the SFRM filter 16 b could be in the iris 14,but it would interfere with iris adjustments, because the iris of zoomlens 10 does not move, as it does in some other lenses.

Referring now to FIGS. 5A and 5B, the same zoom lens 10 is shown as inthe previous figures and it is shown in the same zoom position, i.e.,with a long focal length as in FIGS. 1C, 2, 3C and 4, with the SFRMfilter element 16 b located in the same position as in FIGS. 3C and 4.FIG. 5A shows the focus lens grouping F in a position for focusing on anobject at infinity, while FIG. 5B shows the position for focusing at aminimum distance, namely, with an object at object plane 18 where theon-axis ray tracings T1, T2 meet on the optical axis O and also wherethe off-axis ray tracings T3–T5 meet at a point off the axis O. Again,by comparing the on-axis ray tracings T1, T2 of FIGS. 5A and 5B, it maybe seen that the cross sectional area of the beams of light forming theimage at plane 12 remains substantially constant at the SFRM filter 16 bat the extreme range of focus adjustments, which also is true of thefocus adjustments therebetween, and this is true as well for all thefocal length adjustments as described above.

Also, it should be noted that with lens 10 adjusted to the close focusposition shown in FIG. 5B, whereby it serves as a close focusing(sometimes called macro focusing) lens, the lens essentially functionsas a relay lens to relay the object from object plane 18 at a “finite”distance, as compared to an optically “infinite” distance, to the imageplane 12. This demonstrates that the present invention is applicable toa relay or collecting lens in any optical system, not solely “objective”lenses.

Referring now to FIGS. 6A and 6B, the optical elements of a prime (fixedfocal length) lens 20 are diagrammatically shown for illustrating theincorporation of the present invention in a prime lens. Specifically,the illustrated prime lens 20 is a high performance lens having a fixedfocal length of about 150 mm of the type that has been used for filmcinematography, but is adapted in FIGS. 6A and 6B for HDTV as evidencedby the prisms P1 and P2. FIG. 6A shows the focus lens group F′ of lens20 in the position focused at infinity and FIG. 6B shows the positionfor focusing at the minimum object distance for the lens, whereby FIGS.6A and 6B show the full range of focus adjustments of lens 20. An iris14 is provided in the usual manner and location, and the lens 20 has animage plane 12 at the right hand end, as shown in the Figures. Again, aSFRM filter optical element 16 c is provided on and along the opticalaxis O of lens 20 at an appropriate location. Specifically, at the shownlocation of optical element 16 c within the collecting lens group C, therepresentative on-axis light ray tracings T1 and T2 are nearlycollimated. More importantly for the present invention, the light beamat the SFRM filter 16 c maintains a substantially constant crosssectional area throughout the focusing range of the lens 20, whereby thespatial frequency response remains substantially constant. As with thezoom lens 10 of the previous Figs., the SFRM filter 16 c may be locatedelsewhere in the prime lens 20 if the cross sectional area of the lightbeams formed by the on-axis light rays remains substantially constant.In other prime (fixed focal length) lenses, the SFRM filter opticalelement 16 c may be positioned at a different location that may be morebeneficial, such as where the collimation of the on-axis light beam iseven closer or where, for a lower cost, a smaller diameter of elementmay be used. As with the prior embodiments, it is preferable that theSFRM filter optical element 16 c be removable and replaceable forallowing more versatile uses of the lens 20.

Referring now to FIG. 7, an electronic camera 30 is shown in elevationwith some of the internal optical components diagrammaticallyillustrated. The camera 30 and its optical system are essentially thesame as shown and described in British Patent 2,259,373, except for theinclusion here of the SFRM filter. Specifically, the camera 30 is of atype having a conventional video detector 32, such as a vacuum tube typeor CCD type detector but the camera 30 is adapted to receiveinterchangeable lenses normally used on film cameras. Since the imagecreated by a 35 mm objective lens is larger than the video detector 32is capable of receiving directly, the camera 30 is provided with a lightrelay lens system comprised of relay lens group R1 and relay lens groupR2 that relay the light from the objective lens 34 to the video detector32 and also reduce the size of the image. The objective lens 34, relaylens group R1, relay lens group R2 and video detector 32 are aligned ona optical axis O in that order from adjacent object space (on the leftin FIGS. 7, 8A and 8B) to an image plane 12 (on the right). The opticalaxis is folded at two locations by mirrors for creating a more compactcamera. FIG. 7 also illustrates an operator's viewfinder V that isnormally provided with an electronic camera but that is not relevant tothe present invention.

FIGS. 8A and 8B are optical diagrams of the optical system of FIG. 7with FIG. 8B being an unfolded diagram of the twice-folded opticaldiagram of FIG. 8A and therefore all three Figs. will be describedtogether. Of course, the mirrors that fold the optical axis in FIGS. 7and 8A are omitted from the unfolded optical diagram of FIG. 8B whichotherwise is the same as FIG. 8A. The light from objective lens 34 isdirected to a partial mirror 36 where part of the light passes throughpartial mirror 36 to the optical system of viewfinder V and the balanceof the light is reflected to relay lens group R1 along optical axis O.The light passes through optical lens components 42, 44, 46 and 48 ofrelay lens group R1 and then reflects off full mirror 50 along opticalaxis O to relay lens group R2. The light passes through optical lenscomponents 52, 54, 56 and 58 of relay lens group R2 to the videodetector 32 where the image is formed at image plane 12. In accordancewith the present invention, a SFRM filter 16 d is removably mounted incamera 30 adjacent the lens component 48 within the relay lens group R1.This location for the SFRM filter 16 d is preferred because the on-axislight rays are nearly collimated, as with the above describedembodiments, and the SFRM filter 16 d is of a minimum size for reducingthe cost. There are other locations within the relay lens groups R1 andR2 where the SFRM filter 16 d could be mounted that would produce asatisfactory filtering performance but SFRM filter 16 d would be muchlarger, as may be seen by comparing the size of optical component 48with the size of the other optical components 42, 44, 46, 52, 54, 56 and58 of relay lens groups R1 and R2. Referring more particularly to thelight ray tracings of FIGS. 8A and 8B, the on-axis (zero field angle)tracings T1, T2 are symmetrical about and substantially parallel to theoptical axis at the SFRM filter 16 d and the off-axis (non-zero fieldangle) tracings T3–T5 are substantially parallel to each other but atangles to the surface of SFRM filter 16 d. These light ray tracingsT1–T5 and all other significant light ray tracings that may be developedfor this light relay lens system remain the same through all focusingadjustments of objective lens 34 as well as all focal length adjustmentsif objective lens 34 is a zoom lens. Thus, the spatial frequencyresponse created by SFRM filter 16 d remains constant and can beselected in the same manner as the previously described arrangements ofSFRM filters 16 a, 16 b and 16 c.

The SFRM filter optical elements 16 a, 16 b, 16 c and 16 d (collectively16) shown in the Figures described above may be of any presentlyconventional type or comprised of any other means that modify thespatial frequency response, even though that means may not have beenused previously for that purpose for some reason, but would be usablefor the present invention because of, for example, the location in thelens. In addition to previously conventional diffusion filters that haveparticles, dots, dimples, scratches, lenslets or the like on thesurface(s) of or within a glass plate element or sandwiched between atleast two glass plates forming a glass plate element (or even astretched silk stocking) for diffusing or scattering the light to modifythe spatial frequency, the SFRM filter 16 used in this invention may becomprised of a thin film or films on a glass plate(s), a so-calledphase-only filter, an interference filter that affects the light spatialfrequency response, a holographic element, a controllable and variable,in situ, optical element that can vary the light scattering, direction,phase or the like. Such a controllable optical element may be of anytype or construction, such as but not limited to, electro-opticaldevices controllable by electric current or voltage, acousto-opticaldevices producing electronically controllable sound waves,opto-mechanical devices controllable by movement or physical change,magneto-optical devices controllable by magnetic fields, combinations oftwo or more such devices or other devices, or the like that can apply anexternal operating force for causing variations in the spatial frequencyresponse. Further, more than one SFRM filter 16 may be provided atdifferent locations along the optical axis O, such as but not limited tothe positions shown for SFRM filters 16 a and 16 b, and such multipleSFRM filters 16 may be of the various different types described above.Still further, the spatial frequency response modifying means may beprovided in only a predetermined portion or portions of the crosssectional area of element 16 for causing modification(s) in only aportion or portions of the image at image plane 12. Still further, ifdesired, the SFRM filter 16 also may include other thin films or thelike for causing a desired effect on the colorimetry of the lens system.The consistency of the spatial frequency response produced, i.e., themodulation transfer function, by the present invention allows the use ofthis wide variety of spatial frequency response modifying means,including perhaps the creation of otherwise unattainable visual effects.

Referring now to FIG. 9, a graph of the modulation transfer function(MTF), in general, of various arrangements of a typical lens is shown asa plot of modulation (so-called “contrast”) in percent (%) versus thespatial frequency in cycles/mm at the image plane 12. This graph isshown for the on-axis, zero field angle characteristics but, of course,other graphs can be produced for any position on the image plane 12. Thelong dash line “A” represents a diffraction limit and the short dashline “B” represents the spatial frequency response of a lens without anydiffusion or SFRM filter. The dot-dash line “C” represents the spatialfrequency response of a typical conventional diffusion filter opticalelement 16, whereby certain spatial frequencies, usually the higherfrequencies above 70 cycles/mm, are eliminated which is commonly knownas a high frequency cut-off filter and, for example, will eliminate thefine lines in the face of an actor or actress. The X and dash line “D”represents a spatial frequency response of a thin film or othersophisticated SFRM filter which can reduce the response in a specificrange, such as between about 20–50 cycles/mm as shown, while alsoeliminating the response above 70 cycles/mm as with the conventionaldiffusion filter of line C. The 0 and dash line “E” represents a spatialfrequency response similar to line D but without the high frequencycut-off of the line D. The solid line “F” represents anothersophisticated SFRM filter similar to lines D and E but including areduced modulation at both 30–40 cycles/mm and 60–70 cycles/mm, with anincreased modulation therebetween (40–60 cycles/mm) and no highfrequency cut-off. The dashed lines “G” represent a modification of thespatial frequency response of the SFRM filter of line F by reducing themodulation at 30–40 and 60–70 to nearly zero. A further modification oflines F and G could reduce the modulation to zero over one or both ofthe ranges 30–40 and 6–70 or at other ranges and over larger or smallranges. It should be noted that each of the lines A–F actually convergeor start at 0 (zero) spatial frequency and 100% modulation but a shortportion of each line has been omitted from FIG. 7 for clarity ofillustration of the converging lines. Also, the spatial frequency axisof the graph starts at zero and goes to a maximum frequency that isdetermined by the imaging system aperture and wavelength of light, ascommonly known to those skilled in the art. Essentially, withsophisticated materials and methods, the spatial frequency response canbe modified to produce any desired modulation transfer function desired.When SFRM filters 16 of the type that produce the spatial frequencyresponses of lines C through G, are used in the present invention, thespatial frequency response remains substantially constant over the fullrange of focus adjustments of a prime lens and over the full range offocus and focal length adjustments of a zoom lens, as well as in anyoptical system having at least one adjustable lens element.

Thus, by the present invention, light received by a lens or otherimaging system may be spatial frequency response modified within thelens or other imaging system to produce a predetermined modulationtransfer function of the light reaching the image plane to be receivedby the film, charge coupled device or other detector of a camera orother imaging system to thereby record the desired Modulation TransferFunction for any purpose and may do so instantaneously and in a constantmanner throughout focusing and zooming of the lens. While specificembodiments of this invention have been shown and described, it willreadily appear to those skilled in the art that the invention isapplicable to and may include modifications and other arrangementswithout departing from the invention, whereby this invention is of thefull scope of the appended claims.

1. In a zoom lens system having a variable focal length for use on acamera, an improvement comprising: an optical element positioned on anoptical axis of the zoom lens system at a location along the opticalaxis having a substantially constant cross sectional area of on-axislight beams forming an image at an image plane in the camera throughoutthe variations in focal length; said optical element having means forcausing a modification of the spatial frequency response of lightsupplied to the camera; and said optical element at a location along theoptical axis where the off-axis light rays are substantially collimatedand are at a minimum angle with respect to the axis; wherein saidlocation along the optical axis is adjacent an iris of the lens system.2. The zoom lens system of claim 1, wherein said optical element has asurface that is optically flat, said modifying means being on saidsurface.
 3. The zoom lens system of claim 1, wherein said opticalelement is removable and replaceable from the lens system.
 4. The zoomlens system of claim 3, further including a replacement optical elementhaving substantially the same optical characteristics without means forcausing modification of the spatial frequency response.
 5. The zoom lenssystem of claim 1, wherein said location along the optical axis isadjacent an optical stop of the lens system.
 6. The zoom lens system ofclaim 1, wherein said location along the optical axis is within acollecting lens group of the lens system.
 7. The zoom lens system ofclaim 1, wherein said location along the optical axis has light rayswhich form the on-axis light beams that are substantially perpendicularto said surface of said optical element.
 8. The zoom lens system ofclaim 1, wherein the lens system is an objective lens, and said locationalong the optical axis allows a full range of variable focal lengthchanges of the objective lens without substantially changing said crosssectional area of the on-axis light beams at said optical element. 9.The zoom lens system of claim 1, wherein said optical element iscomprised of a diffusion filter.
 10. The zoom lens system of claim 9,wherein said diffusion filter optical element has a surface with atleast one of dispersive particles, spaced and opaque dots, spaceddimples, lenslets, and scratches.
 11. The zoom lens system of claim 1,wherein said modifying means of said optical element is comprised of aholographic element.
 12. The zoom lens system of claim 1, wherein saidmodifying means of said optical element is comprised of at least onethin film on at least one surface of said optical element.
 13. The zoomlens system of claim 1, wherein said modifying means of said opticalelement has properties which are variable in spatial frequency responsemodification as a result of variations in an external operative forceapplied to said optical element.
 14. The zoom lens system of claim 13,wherein said external operative force is electrical.
 15. The zoom lenssystem of claim 13, wherein said external operative force is acoustical.16. The zoom lens system of claim 13, wherein said external operativeforce is mechanical.
 17. The zoom lens system of claim 13, wherein saidexternal operative force is optical.
 18. The zoom lens system of claim13, wherein said external operative force is magnetic.
 19. The zoom lenssystem of claim 13, wherein said external operative force is acombination of at least two of electrical, acoustical, mechanical,optical and magnetic external operative forces.
 20. The zoom lens systemof claim 1, wherein said spatial frequency modifying means is providedonly in a portion of said substantially constant cross sectional area ofon-axis light beams passing through said optical element.
 21. The zoomlens system of claim 1, wherein said spatial frequency modifying meansis provided in at least two separate portions of said substantiallyconstant cross sectional area of on-axis light beams passing throughsaid optical element.
 22. In a zoom lens system, an improvementcomprising; an optical spatial frequency response modifying filterpositioned on an optical axis of the lens system at an intermediatelocation along the optical axis within the lens, said intermediatelocation having a substantially constant on-axis light beam crosssectional area and substantially collimated light for any modes ofadjustment of the lens system; and said intermediate location havingsubstantially collimated off-axis light rays at a minimum angle withrespect to the axis for any modes of adjustment of the lens system;wherein said intermediate location along the optical axis is adjacent aniris of the lens system.
 23. The zoom lens system of claim 22, whereinthe lens system comprises an objective lens having focus adjustingmeans, and said intermediate location maintains the substantiallyconstant on-axis light beam cross sectional area and the substantiallycollimated off-axis light rays are at a minimum angle with respect tothe axis during focus adjustments.
 24. The zoom lens system of claim 22,wherein the lens system comprises an objective lens having zooming meansfor varying the focal length, and said intermediate location maintainsthe substantially constant on-axis light beam cross sectional area andthe substantially collimated off-axis light rays are at a minimum anglewith respect to the axis during zooming of the lens system.
 25. The zoomlens system of claim 22, wherein the lens system comprises a light relaylens system, and said intermediate location is within said light relaylens system.
 26. The zoom lens system of claim 22, 23, 24 or 25, whereinsaid optical element is removable and replaceable from the lens system.27. The zoom lens system of claim 26, further including a replacementoptical element having substantially the same optical characteristicsand without means for modifying the spatial frequency response.
 28. Thezoom lens system of claim 22, 23, 24 or 25, wherein said intermediatelocation along the optical axis is adjacent an optical stop of the lenssystem.
 29. The zoom lens system of claim 22, 23, 24 or 25, wherein saidlocation along the optical axis is within a collecting lens group of thelens system.
 30. The zoom lens system of claim 22, 23, 24 or 25, whereinsaid intermediate location along the optical axis is where the on-axislight beam cross sectional area is substantially the smallest.
 31. Thezoom lens system of claim 30, wherein said location along the opticalaxis is where the off-axis light rays from an object point aresubstantially parallel to each other and are at a minimum angle withrespect to the axis.
 32. The zoom lens system of claim 22, 23, 24 or 25,wherein said filter is comprised of an optical glass element having atleast one of dispersive particles, spaced and opaque dots, spaceddimples, lenslets, and scratches.
 33. The zoom lens system of claim 22,23, 24 or 25, wherein said filter is comprised of a semi-transparentfabric.
 34. The zoom lens system of claim 22, 23, 24 or 25, wherein saidfilter is comprised of a holographic element.
 35. The zoom lens systemof claim 22, 23, 24 or 25, wherein said filter is comprised of anoptical glass element having at least one thin film for modifying thespatial frequency response of light.
 36. The zoom lens system of claim22, 23, 24 or 25, wherein said filter is comprised of an optical elementhaving properties which are variable in spatial frequency response as aresult of variations in an external operative force applied to saidoptical element.
 37. The zoom lens of claim 36, wherein said externaloperative force is electrical.
 38. The zoom lens system of claim 36,wherein said external operative force is acoustical.
 39. The zoom lenssystem of claim 36, wherein said external operative force is mechanical.40. The zoom lens system of claim 36, wherein said external operativeforce is optical.
 41. The zoom lens system of claim 36, wherein saidexternal operative force is magnetic.
 42. The zoom lens system of claim36, wherein said external operative force is a combination of at leasttwo of electrical, acoustical, mechanical, optical and magnetic externaloperative forces.
 43. The zoom lens system of claim 22, 23, 24 or 25,wherein said filter is provided with spatial frequency modifying meansonly in a portion of the substantially constant on-axis light beamcross-sectional area of light passing through said filter.
 44. The zoomlens system of claim 22, 23, 24 or 25 wherein said filter is providedwith spatial frequency modifying means in at least two separate portionsof the substantially constant on-axis light beam cross-sectional area oflight passing through said filter.